1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus in which a transfer device has an improved separation/contact mechanism.
2. Description of the Related Art
A tandem type image forming apparatus is known as a kind of image forming apparatus such as a copier or printer. In the tandem type image forming apparatus, toner images are formed respectively on plural photosensitive drums arranged in parallel. The toner images are transferred to a paper sheet with the toner images multi-layered on the paper sheet, to form a color image. Even though plural photosensitive drums are provided in such a tandem type image forming apparatus, the whole image forming apparatus is demanded to be compact in recent years. An image forming apparatus has a quadruple tandem type image forming mechanism capable of printing color and monochrome images and employs an intermediate transfer belt. In this image forming apparatus, transfer drums to be rotated are selectively switched between a case of printing a color image and another case of printing a monochrome image from the perspective of extending lifetime of the apparatus. When printing a monochrome image, only one transfer drum for monochrome images is rotated, and transfer drums for color images are stopped. In this case, the intermediate transfer belt is obviously rotated, therefore the intermediate transfer belt and the transfer drums not rotated are maintained apart from each other. At this time, corresponding primary transfer rollers and the belt are also maintained apart from each other. During a standby period for printing, all the transfer drums and the belt are maintained apart from each other so as to prevent deformation of the primary transfer rollers which use sponges as materials. According to known techniques, the whole belt unit is moved in order to move the transfer drums into contact with and a part from a belt, and a large-scale mechanism is hence required.
There have been several proposals concerning contact/separation between transfer drums and an intermediate transfer belt. For example, there has been proposed a separation mechanism for a transfer device which steadily operates and prevents noise and vibration (for example, see Jpn. Pat. Appln. Laid-Open Publication No. 2004-117499). This mechanism relates to a transfer device in which a sheet transfer roller as a transfer member is moved into contact with and apart from an intermediate transfer belt. The transfer device has a cam mechanism for moving the sheet transfer roller into contact with and apart from the intermediate transfer belt. When a small diameter side of a contact/separation cam faces the sheet transfer roller, the sheet transfer roller is moved so as to make contact with the intermediate transfer belt. A cam shaft is applied with a heavier load than torque which the cam shaft receives due to stress acting on the contact/separation cam when the sheet transfer roller makes contact with the intermediate transfer belt.
There has also been a proposal to relax an impact which occurs when a transfer roller makes contact (for example, see Jpn. Pat. Appln. Laid-Open Publication No. 2005-91725). According to this proposal, an image forming apparatus has an intermediate transfer member and a transfer roller. A toner image formed on a photosensitive member is transferred to the intermediate transfer member while the intermediate transfer member rotates plural times. The transfer roller is provided to be able to freely move into contact with and apart from the intermediate transfer member. The transfer roller transfers the toner image on the intermediate transfer member to a sheet material while conveying the sheet material pressed against to the intermediate transfer member by the transfer roller. The image forming apparatus further has an axle provided to be rotatable, two cams provided on the axle, and push arms respectively having ends in contact with the cams and other ends in contact with rotation bearings located at ends of the transfer roller. Phases of the two cams are shifted relative to each other so that timing when the transfer roller transits from a separate state of being separate from the intermediate transfer member to a contact state comes earlier at one end of the transfer roller in the lengthwise direction of the roller than at the other end of the transfer roller.
In known contact/separation mechanisms, the whole belt unit is moved to make contact to and separate from transfer drums. Consequently, a large-scale mechanism is required.
In addition, since the whole belt unit is moved, a high output motor, a large-scale contact/separation mechanism, and the like are necessary. Furthermore, since such a large unit is moved into contact with and separate from transfer drums, contact impact affected on the transfer drums is strong. As a result, the transfer drums are damaged, e.g., abraded due to repetitive impact if the image forming apparatus is used for a long time.